1. Field of the Invention
Embodiments of the invention relate to switching power supply devices that receive a rectified AC voltage, and deliver a stable DC voltage, and in particular to switching power supply devices that perform power factor correction.
2. Description of the Related Art
A power factor correction (PFC) converter of a boost chopper type, a type of switching power supply device, generates a stable DC output voltage while generating an input current proportional to an input voltage utilizing self-excited oscillation of an inductor. The PFC converter has excellent characteristics including small-size, high efficiency, and production at a low cost. FIG. 9 shows a schematic construction of this type of switching power supply device, which is a power factor correction converter, in which the reference symbol BD designates a rectifier circuit that rectifies an AC power given from an AC power supply through an input filter F and delivers the rectified voltage to the switching power supply device.
This switching power supply device is provided with an inductor L connected to the rectifier circuit BD and a switching element Q that forms a current path from the rectifier circuit BD through the inductor L in the ON state of the switching element Q. The switching power supply device is further provided with a diode D that forms a current path between the inductor L and an output capacitor C2 in the OFF state of the switching element Q and a control circuit CONT that ON/OFF drives the switching element Q to control the current flowing through the inductor L. The symbol C1 designates an input capacitor.
The switching element Q provides the inductor L with a current proportional to the input voltage applied to the switching power supply device in the ON state of the switching element Q. The current flowing through the inductor L, inductor current, rises from zero over the ON period of the switching element Q. Upon turning OFF of the switching element Q, the voltage across the inductor L changes its polarity and the inductor current is delivered to the output capacitor C2 in the output side through the diode D. When the inductor current becomes zero, the switching element Q is turned ON again to transfer to the next cycle. This procedure is repeated.
The control circuit CONT basically conducts two functions: one of them is determination of an ON width Ton of the switching element Q corresponding to the difference between the output voltage Vo obtained across the output capacitor C2 and a predetermined target output voltage and turning OFF of the switching element Q in a voltage mode; the other is zero current switching, which performs detecting zero current through the inductor and turning ON of the switching element Q.
The input voltage Vi given to the inductor L has a voltage waveform varying over a half period of a sine wave. In the ON/OFF control of the switching element Q in the voltage mode, however, an ON width Ton of the switching element Q is basically constant irrespective of the phase of the input voltage Vi. As a result, at a low voltage phase where the instantaneous value of the input voltage Vi is low, the current supplied to the inductor L is insufficient, which increases so-called dead angle, a reactive voltage phase, lowering a power factor.
To improve the power factor, it has been proposed in U.S. Pat. No. 6,984,963 (also referred to herein as “Patent Document 1”) and U.S. Pat. No. 7,116,090 (also referred to herein as “Patent Document 2”) to broaden the ON width Ton of the switching element Q in the phase of low instantaneous value of the input voltage Vi than in a high voltage phase where the instantaneous value of the input voltage Vi is high. In the method disclosed in Patent Document 1, the ON width Ton of the switching element Q is adjusted based on the phase information of the input voltage Vi obtained by detecting the peak value of the inductor current, which is proportional to the input voltage Vi. In the method disclosed in Patent Document 2, the ON width Ton of the switching element Q is adjusted based on the phase information of the input voltage Vi obtained by detecting the differential value of the inductor current, which is also proportional to the input voltage Vi.
In the both methods disclosed in Patent Documents 1 and 2, the phase information of the input voltage Vi is obtained by inserting a detecting resistor for inductor current detection in the current path including the inductor L and detecting the inductor current from the voltage drop across the detecting resistor. The resistance value of the detecting resistor is determined from the specifications, or circuit constants, of the switching power supply device including the inductance value of the inductor L and the output voltage Vo. The resistance value changes the amount of compensation, or adjustment width, of the ON width Ton. However, in a phase of low instantaneous value of input voltage Vi, where the current flowing in the inductor L is small, the accuracy of current detection using the detecting resistor decreases and the phase information of the input voltage Vi is hardly detected with high accuracy. Moreover, noises adversely affect the current detection since the inductor current itself is low in proportion to the input voltage Vi.
Thus, as has been described, there exists certain shortcomings in the related art.